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Thursday, September 2, 2010

Insect Head

The Head

The head, being the anterior tagma, bears the major sense organs and the mouth parts. Considerable controversy still surrounds the problem of segmentation of the insect head, especially concerning the number and nature of segments anterior to the mouth. At various times it has been argued that there are from three to seven segments in the insect head, though it is now widely agreed that there are six.

The embryological studies have demonstrated convincingly that an acron is present. However, it is never seen in fossil insects or other arthropods because it moved dorsally to merge imperceptibly into the region between the compound eyes. Both embryology and paleontology have conﬁrmed that there are three preoral and three postoral segments The ﬁrst preoral segment is preantennal; it is called the protocerebral clypeolabral segment. The segment itself has disappeared but its appendages remain as the clypeolabrum. The second preoral (antennal/deutocerebral) segment bears the antennae which are therefore true segmental appendages. The third preoral (intercalary/tritocerebral) segment appears brieﬂy during embryogenesis, then is lost. Its appendages, however, remain as part of the hypopharynx (Kukalova´-Peck, 1992). Head segments 4–6 are post-oral and named the mandibular maxillary, and labial, respectively. Their appendages form the mouthparts from which their names are derived. In addition, the sternum of the mandibular segment becomes part of the hypopharynx.

General Structure

Primitively the head is oriented so that the mouthparts lie ventrally (the hypognathous condition) (Figure 3.3B). In some insects, especially those that pursue their prey or use their mouthparts in burrowing, the head is prognathous in which the mouthparts are directed anteriorly (Figure 3.4A). In many Hemiptera the suctorial mouthparts are posteroventral in position (Figure 3.4B), a condition described as opisthognathous (opisthorhynchous).The head takes the form of a heavily sclerotized capsule, and only the presence of the antennae and mouthparts provides any external indication of its segmental construction. In most adult insects and juvenile exopterygotes a pair of compound eyes is situated dorsolater- ally on the cranium, with three ocelli between them on the anterior face (Figure 3.3A). The two posterior ocelli are somewhat lateral in position; the third ocellus is anterior and median. The antennae vary in location from a point close to the mandibles to a more median position between the compound eyes. On the posterior surface of the head capsule is an aperture, the occipital foramen, which leads into the neck. Of the mouthparts, the labrum hangs down from the ventral edge of the clypeus, the labium lies below the occipital foramen, and the paired mandibles and maxillae occupy ventrolateral positions (Figure 3.3B). The mouth is situated behind the base of the labrum. The true ventral surface of the head capsule is the hypopharynx (Figure 3.3D), a membranous lobe that lies in the preoral cavity formed by the ventrally projecting mouthparts.There are several grooves and pits on the head (Figure 3.3A–C), some of which, by virtue of their constancy of position within a particular insect group, constitute important taxonomic features. The grooves are almost all sulci. The postoccipital sulcus separates the maxillary and labial segments and internally forms a strong ridge to which are attached the muscles used in moving the head and from which the posterior arms of the tentorium arise (see following paragraph). The points of formation of these arms are seen externally as deep pits in the postoccipital groove, the posterior tentorial pits The epicranial suture is a line of weakness occupying a median dorsal position on the head. It is also known as the ecdysial line, for it is along this groove that the cuticle splits during ecdysis. In many insects the epicranial suture is in the shape of an inverted Y whose arms diverge above the median ocellus and pass ventrally over the anterior part of the head. The occipital sulcus which is commonly found in orthopteroid insects, runs transversely across the posterior part of the cranium. Internally it forms a ridge that strengthens this region of the head. The subgenal sulcus isa lateral groove in the cranial wall running slightly above the mouthpart articulations. That part of the subgenal sulcus lying directly above the mandible is known as the pleurostomal sulcus; that part lying behind is the hypostomal sulcus, which is usually continuous with the postoccipital suture. In many insects the pleurostomal sulcus is contin- ued across the front of the cranium (above the labrum), where it is known as the Epistomal ( frontoclypeal) sulcus. thin this sulcus lie the anterior tentorial pits, which indicate the internal origin of the anterior tentorial arms. The antennal and ocular sulci indicate internal cuticular ridges bracing the antennae and compound eyes, respectively. A subocular sulcus running dorsolaterally beneath the compound eye is often present. The tentorium (Figure 3.5) is an internal, cranial-supporting structure whose morphology varies considerably among different insect groups. Like the furca of the thoracic segments (Section 4.2), with which it is homologous, it is produced by invagination of the exoskeleton. Generally, it is composed of the anterior and posterior tentorial arms that may meet and fuse within the head. Frequently, additional supports in the form of dorsal arms are found. The latter are secondary outgrowths of the anterior arms and not apodemes. The junction of the anterior and posterior arms is often enlarged and known as the tentorial bridge corporotentorium In addition to bracing the cranium, the tentorium is also a site for the insertion of muscles controlling movement of the mandibles, maxillae, labium, and hypopharynx.The grooves described above delimit particular areas of the cranium that are useful in descriptive or taxonomic work. The major areas are as follows. The frontoclypeal area is the facial area of the head, between the antennae and the labrum. When the epistomal sulcus is present, the area becomes divided into the dorsal frons and the ventral clypeus. The latter is often divided into a postclypeus and an anteclypeus The vertex is the dorsal surface of the head. It is usually delimited anteriorly by the arms of the epicranial suture and posteriorly by the occipital sulcus. The vertex extends laterally to merge with the gena, whose anterior, posterior, and ventral limits are the subocular, occipital, and subgenal sulci, respectively. The horseshoe-shaped area lying between the occipital sulcus and postoccipital sulcus is generally divided into the dorsal occiput, which merges laterally with the postgenae. The postocciput is the narrow posterior rim of the cranium surrounding the occipital foramen.It bears a pair of occipital condyles to which the anterior cervical sclerites are articulated. Below the gena is a narrow area, the subgena, on which the mandible and maxilla are articulated. The labium is usually articulated directly with the neck membrane (Figure 3.3C), but in some insects a sclerotized region separates the two. This sclerotized area develops in one of three ways: as extensions of the subgenae which fuse in the midline to form a subgenal bridge, as extensions of the hypostomal areas to form a hypostomal bridge, or(in most prognathous heads) through the extension ventrally and anteriorly of a ventral cervical sclerite to form the gula. At the same time the basal segment of the labium may also become elongated (Figure 3.4A).

FIGURE 3.4. (A) Prognathous; and (B) opisthognathous types of head structure. [A, from R. E. Snodgrass, Principles of Insect Morphology Copyright 1935 by McGraw-Hill, Inc. Used with permission of McGraw-Hill Book Company. B, after R. F. Chapman, 1971, The Insects: Structure and Function By permission of Elsevier North-Holland, Inc., and the author.]

FIGURE 3.5. Relationship of the tentorium to grooves and pits on the head. Most of the head capsule has been cut away. [From R. E. Snodgrass. Principles of Insect Morphology Copyright 1935 by McGraw-Hill, Inc. Used with permission of McGraw-Hill Book Company.]